The present invention relates to a control circuit for varying the magnitude of a holding current in an electromagnetic device with a coil where the electromagnetic device is actuated with an actuating current and held in an operative condition by the holding current.
A coil of wire forms an inductor. An inductor resists changes in current. They store energy in the form of a magnetic field that is produced by the current passing through the inductor. Any change in this current induces a voltage that opposes the change in the current. The inductance of the inductor is proportional the number of turns in the coil and the permeability of the material surrounding the coil. Permeability is the ability of a materiel to concentrate magnetic flux. Higher permeability magnetic materials result in higher inductance values and lower permeability materials result in lower inductance. Air has a permeability of one and iron materials have higher permeability.
Inductors have properties of inductance and resistance. The DC resistance of the coil is determined by the resistance of the wire used in the coil. The number of turns in the coil and the Permeability of the media surrounding the coil determine the inductance.
The current in an inductor rises exponentially when a fixed DC voltage is applied across its terminals. Ohm""s law determines the steady state current given a value of the DC voltage and the resistance of the coil. The initial current is zero, and its rate of change is determined by the inductance of the coil. Closure of the switch in FIG. 1 results in the exponential current profile shown in FIG. 2. The equation for this response is:       i    ⁡          (      t      )        =            v      R        ⁢          (              1        -                  e                                    -              t                        ⁢                          R              L                                          )      
where:
v is the DC voltage across the coil
R is the resistance of the coil
L is the inductance of the coil
i is the current through the coil
t is time from the application of v.
A relay, solenoid and magnetic clutch are constructed using an electrical magnet that actuates an armature to initiate a function. The relay armature when actuated opens or closes electrical contacts. The solenoid when actuated effects some movement to engage or disengage a mechanism. The actuation of the armature in these devices is caused by the magnetic field developed by current passing through a coil of wire. Normally the coil is wound on a spool that is then placed within a magnet core that concentrates the magnetic field. This core has a gap in the magnetic path where the armature is located. Here is where the magnetic field attracts the armature and the armature moves in an effort close the gap.
FIG. 3 shows a relay coil with the spool an core and mounting bracket and an armature. The gap is between the armature and the pole peace on the top of the spool. When sufficient current flows through the coil the armature is pulled down to the pole piece in a manner indicated by the arrow in FIG. 3.
Constructing a simple circuit such as in FIG. 1 and plotting the current from the battery results in a response similar to that of FIG. 2. That is until the magnetic field has grown strong enough to acute the armature. The actuation of the armature closes the gap in the magnetic field changing the effective permeability of the core resulting in a change in inductance. This disrupts the current vs. time profile. FIG. 4 below is a plot of actual data from a Potter and Blumfield KUP14D15 relay. The triangles mark the actual data and the lines are best-fit curves to those triangles. There are two curves, the one to the left is for 6.1 mh (milli-Henry) which is the inductance of the relay coil with the armature open and the right one is for 6.3 mh and is the inductance with the armature closed. These inductances were estimated by fitting the inductive charging equation to the data. xe2x80x9cAppendix Bxe2x80x9d shows the worksheets used to fit this data.
The circled area in FIG. 4 is the area in which actuation accrued. During the time before the circle the armature is attracted by the magnetic field, but its strength is insufficient to overcome the force holding the armature in the open position. Inside the circle is when the armature moves from open to closed. To the right of the circle the armature is in the closed position. Interrupting the current flow will reverse this process and the armature moving to its open position will introduce a similar disruption to the response.
The prior art includes control circuits for both stepper motors and solenoids:
U.S. Pat. No. 5,744,922 to Neary et al. (xe2x80x9cNearyxe2x80x9d) discloses a current regulator, the current regulator utilizing a well known driver chip. The driver chip has an input signal known as a brake signal xe2x80x9cBRKxe2x80x9d which typically is used to stop a standard DC motor. In the disclosed embodiment of Neary, the brake signal xe2x80x9cBRKxe2x80x9d is used to create a low resistance current path in order to sustain the current of the current regulator which is used in conjunction with a stepper motor.
U.S. Pat. No. 4,536,818 to Nielsen (xe2x80x9cNielsenxe2x80x9d) discloses a solenoid driver circuit that reduces power consumption by switching a corresponding solenoid coil current during a decay period from an initial peak current to a lower magnitude sustaining peak current. Current decays from the sustaining peak current magnitude for a predetermined length of time to a lower current level. Two transistors and a Zener diode are operatively connected to the solenoid and controlled by a logic circuit to apply the desired current to the solenoid. A sense resistor is coupled in series with the solenoid to sense current in the solenoid. The Zener diode is coupled in parallel with the sense resistor to provide a current decay path from the solenoid parallel to the sense resistor. The two transistors are turned on and off using logic flip-flops to sense voltage comparisons with the initial peak current voltage, the sustaining peak current, and the sustaining low current. A logic signal is generated as a function of the predetermined length of time, and an output signal is coupled to the bases of the two transistors to control their on/off states.
While the stepper motor control approach of Neary is well suited for controlling a stepper motor, it does not appear to be as well suited for controlling an electromagnetic device, such as a solenoid, relay or clutch, where the device is operated at a current that is significantly lower than its xe2x80x9cactivationxe2x80x9d current. Essentially the stepper motor operates at activation current IRef. This approach is inefficient for an electromagnetic device that need not operate at its activation current for significant time periods.
While the control circuit of Nielsen is well suited for use with a solenoid, it does not appear to function optimally as an on/off switch, such as a switch for unlocking a CD-ROM player or automobile door. More particularly, Nielsen is directed toward transmission control and seeks to obtain a means for reducing power dissipation and minimizing nonlinearity in solenoid output in response to an input having a duty cycle.
In view of the above, there is a need for an efficient electronic switch that can be used to xe2x80x9ccrisplyxe2x80x9d switch a solenoid, relay, clutch or the like from an on state to an off state with a minimum amount of power dissipation.
The disclosures of Neary and Nielsen are incorporated herein by reference.
In one embodiment of the disclosed invention, there is provided a control circuit for use with an electromagnetic device with a coil. In practice, the electromagnetic device is actuated with an actuating current and held in an operative condition by a holding current with the holding current being significantly lower in magnitude than the actuating current. The control circuit comprises: a first transistor disposable in one of an off state and an on state, said first transistor communicating with said coil; a second transistor disposable in one of an off state and an on state, said second transistor communicating with said coil, wherein during a powered mode, the first transistor is disposed in the on state and the second transistor is disposed in the off state, and, during a shorted mode, the first transistor is disposed in the off state and the second transistor is disposed in the on state; a power source selectively communicating with the coil, wherein, during a first time interval, said power source communicates with said coil in the powered mode and, during a second time interval, said power source is disconnected from said coil in the shorted mode so that current is recirculated by said second transistor; and a switching actuator communicating with said first and second transistors, said switching actuator operating cooperatively with the first and second transistors to transition the holding current from a first magnitude to a second magnitude while maintaining the electromagnetic device in the operative condition.